In a laser, the equilibrium and non-equilibrium conditions of an atom refer to its energy state. Let's explore each condition:
Equilibrium condition: In the equilibrium condition, atoms within a laser system are at thermal equilibrium with their surroundings. This means that the energy distribution among the atoms follows the principles of thermodynamics, particularly the Boltzmann distribution. Atoms in their equilibrium state occupy various energy levels according to their respective statistical weights, which depend on temperature. The equilibrium condition is typically associated with low energy and a lack of population inversion.
Non-equilibrium condition: In a laser, a non-equilibrium condition is established to achieve population inversion, which is a prerequisite for laser operation. Population inversion occurs when a larger number of atoms or molecules are in an excited state (higher energy level) than in the ground state (lower energy level). This condition is essential for the amplification of light in a laser.
To create a non-equilibrium condition, a process called pumping is employed. Pumping involves energizing the laser medium (atoms, ions, or molecules) by external means such as optical pumping, electrical discharge, or chemical reactions. The pumping process provides energy to the atoms, enabling them to populate higher energy levels and reach a state of population inversion. Once population inversion is achieved, spontaneous and stimulated emissions can occur, leading to the coherent and amplified light output characteristic of a laser.
In summary, the equilibrium condition refers to the thermal equilibrium of atoms with their surroundings, while the non-equilibrium condition involves population inversion achieved through external pumping to enable laser operation.